Barrier regions for image sensors

ABSTRACT

Embodiments of the invention provide a barrier region for isolating devices of an image sensor. The barrier region comprises a charge accumulation region of a particular conductivity type in a substrate electrically connected to a voltage source terminal. The charge accumulation region is adjacent to at least one pixel cell of a pixel array. The charge accumulation region accumulates charge and prevents charge transference from a pixel cell or peripheral circuitry on one side of the barrier region to a pixel cell on another side of the barrier region.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional application of U.S. patentapplication Ser. No. 10/768,652, filed on Feb. 2, 2004, now U.S. Pat.No. 7,002,231 the disclosure of which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductor devices,particularly to improved isolation techniques for image sensors.

BACKGROUND OF THE INVENTION

An image sensor generally includes an array of pixel cells. Each pixelcell includes a photo-conversion device for converting light incident onthe array into electrical signals. An image sensor also typicallyincludes peripheral circuitry for controlling devices of the array andfor converting the electrical signals into a digital image.

FIG. 1 is a top plan view block diagram of a portion of a typical CMOSimage sensor 10, while FIG. 2A is a top plan view of one pixel cell 20and FIG. 2B is a schematic diagram of the pixel cell 20. Image sensor 10includes an array 11 of pixel cells 20. The pixel cells 20 are arrangedin columns and rows (not shown). The array 11 includes pixel cells 20 inan active array region 12 and pixel cells 20 in a black region 13. Theblack region 13 is similar to the active array region 12, except thatlight is prevented from reaching the photo-conversion devices of thepixel cells 20 in the black region 13 by, for example, a metal layer, ablack color filter array, or any opaque material (not shown). Signalsfrom pixel cells of the black region 13 can be used to determine theblack level for the array 11, which is used to adjust the resultingimage produced by the image sensor 10.

FIG. 2A is a top plan view of a conventional pixel cell 20. FIG. 2B is aschematic diagram of the pixel cell of FIG. 2A. As is known in the art,a pixel cell 20 functions by receiving photons of light and convertingthose photons into charge carried by electrons. For this, each one ofthe pixel cells 20 includes a photo-conversion device 21, which is shownas a photodiode, but can be a photogate, photoconductor, or otherphotosensitive device. The photodiode 21 includes a photodiode chargeaccumulation region 22 and a p-type surface layer 24.

Each pixel cell 20 also includes a transfer transistor 27 fortransferring charge from the photodiode charge accumulation region 22 toa floating diffusion region 25 and a reset transistor 28, for resettingthe diffusion region 25 to a predetermined charge level, Vaa-pix, priorto charge transfer. The pixel cell 20 also includes a source followertransistor 29 for receiving and amplifying a charge level from thediffusion region 25 and a row select transistor 26 for controlling thereadout of the pixel cell 20 contents from the source followertransistor 29. As shown in FIG. 2A, the reset transistor 28, sourcefollower transistor 29 and row select transistor 26 include source/drainregions 60.

Several contacts 23 provide electrical connections for the pixel cell20. For example, as shown in FIG. 2, a source/drain region of the resettransistor 28 is electrically connected to an array voltage sourceterminal, Vaa-pix through a first contact 23; the gate of the sourcefollower transistor is connected to the floating diffusion region 25through a second contact 23; and an output voltage Vout is output fromthe pixel cell 20 through a third contact 23.

Referring again to FIG. 1, after pixel cells of array 11 generate chargein response to incident light, electrical signals indicating chargelevels are read out and processed by circuitry 15 peripheral to array11. Peripheral circuitry 15 typically includes row select circuitry 16and column select circuitry 17 for activating particular rows andcolumns of the array 11; and other peripheral circuitry 18, which caninclude analog signal processing circuitry, analog-to-digital conversioncircuitry, and digital logic processing circuitry. Peripheral circuitry15 can be located adjacent to the array 11 as shown in FIG. 1.

In order to obtain a high quality image, it is important that theperipheral circuitry 15 not interfere with the pixel cells 20 of thearray 11. During operation, the peripheral circuitry 15 can generatecharge carriers, e.g., electrons. If the peripheral circuitry 15 isadjacent to the array 11, electrons generated by the peripheralcircuitry 15 can travel to and interfere with array pixel cells 20,especially those pixel cells 20 on the edges of the array 11 adjacentthe peripheral circuitry 15. The interfering electrons aremisinterpreted as a true pixel signal and image distortion can occur.

Another problem encountered in the conventional image sensor 10 isinterference from the active array region 12 with the black region 13.When very bright light is incident on active region 12 pixel cells 20adjacent to the black region 13, blooming can occur and excess chargefrom the active region 12 pixel cells 20 can travel to and interferewith pixel cells 20 in the adjacent black region 13. This can causeinaccurate black levels and distortion of the resultant image.

Accordingly, it would be advantageous to have an improved image sensorwith reduced interference.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the invention provide a barrier region for isolatingdevices of an image sensor. The barrier region comprises a chargeaccumulation region of a particular conductivity type in a substrateelectrically connected to a voltage source terminal. The chargeaccumulation region is adjacent to at least one pixel cell of a pixelarray. The charge accumulation region accumulates charge and preventscharge transference from a pixel cell or peripheral circuitry on oneside of the barrier region to a pixel cell on another side of thebarrier region.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages and features of the invention willbecome more apparent from the detailed description of exemplaryembodiments provided below with reference to the accompanying drawingsin which:

FIG. 1 is a top plan view block diagram of a conventional image sensor;

FIG. 2A is a top plan view of a conventional CMOS pixel cell;

FIG. 2B is a schematic diagram of the pixel cell of FIG. 2A;

FIGS. 3A and 3B are top plan view block diagrams of image sensorsaccording to exemplary embodiments of the invention;

FIG. 4A is a top plan view of a barrier region according to an exemplaryembodiment of the invention;

FIG. 4B is a top plan view of a barrier region according to anotherexemplary embodiment of the invention;

FIG. 4C is a top plan view of a barrier region according to anotherexemplary embodiment of the invention;

FIG. 4D is a top plan view of a barrier region according to anotherexemplary embodiment of the invention;

FIG. 5A depicts the formation of the barrier region of FIG. 4A at aninitial stage of processing;

FIGS. 5B-5H depict the formation of the barrier region of FIG. 4A atintermediate stages of processing; and

FIG. 6 is a block diagram of a processor system according to anexemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof and illustrate specificembodiments in which the invention may be practiced. In the drawings,like reference numerals describe substantially similar componentsthroughout the several views. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention, and it is to be understood that other embodiments may beutilized, and that structural, logical and electrical changes may bemade without departing from the spirit and scope of the presentinvention.

The terms “wafer” and “substrate” are to be understood as includingsilicon, silicon-on-insulator (SOI), or silicon-on-sapphire (SOS)technology, doped and undoped semiconductors, epitaxial layers ofsilicon supported by a base semiconductor foundation, and othersemiconductor structures. Furthermore, when reference is made to a“wafer” or “substrate” in the following description, previous processsteps may have been utilized to form regions or junctions in the basesemiconductor structure or foundation. In addition, the semiconductorneed not be silicon-based, but could be based on silicon-germanium,germanium, or gallium-arsenide.

The term “pixel” or “pixel cell” refers to a picture element unit cellcontaining a photo-conversion device for converting electromagneticradiation to an electrical signal. Typically, the fabrication of allpixel cells in an image sensor will proceed concurrently in a similarfashion.

Referring to the drawings, FIG. 3A depicts an image sensor 300 aaccording to an exemplary embodiment of the invention. Image sensor 300a includes a pixel array 311 comprising an active array region 12 andblack region 13. There is also peripheral circuitry 15 adjacent to thearray 311. As shown in FIG. 3, the peripheral circuitry can include rowselect circuitry 16 and column select circuitry 17 for activatingparticular rows and columns of the array 11; and other peripheralcircuitry 18, which can include analog signal processing circuitry,analog-to-digital conversion circuitry, and digital logic processingcircuitry. The configuration of image sensor 300 a is exemplary only.Accordingly, image sensor 300 a need not include peripheral circuitry 15adjacent to the array 311 and/or array 311 need not include black region13.

FIG. 3B depicts an image sensor 300 b according to another exemplaryembodiment of the invention. The image sensor 300 b is similar to theimage sensor 300 a, except that image sensor 300 b includes additionalblack regions 313 a and 313 b. Like black region 13, the black regions313 a and 313 b also include pixel cells 20 where light is preventedfrom reaching the photo-conversion devices of the pixel cells 20 in theblack regions 313 a and 313 b by, for example, a metal layer, a blackcolor filter array, or any opaque material (not shown). The blackregions 313 a and 313 b operate in the same manner as black region 13.Therefore, without isolation the black regions 313 a and 313 b canexperience interference from pixel cells of active array region 12 orperipheral circuitry 15.

Illustratively, image sensors 300 a and 300 b are CMOS image sensors andarray 311 includes CMOS pixel cells 20. It should be noted, however,that embodiments of the invention include CCD image sensors. In such acase, array 311 would instead include pixel cells and peripheralcircuitry suitable for a CCD image sensor.

It should be further noted that the configuration of pixel cell 20 isonly exemplary and that various changes may be made as are known in theart and pixel cell 20 may have other configurations. Although theinvention is described in connection with a four-transistor (4T) pixelcell 20, the invention may also be incorporated into other pixelcircuits having different numbers of transistors. Without beinglimiting, such a circuit may include a three-transistor (3T) pixel cell,a five-transistor (5T) pixel cell, a six-transistor (6T) pixel cell, anda seven-transistor pixel cell (7T). A 3T cell omits the transfertransistor, but may have a reset transistor adjacent to a photodiode.The 5T, 6T, and 7T pixel cells differ from the 4T pixel cell by theaddition of one, two, or three transistors, respectively, such as ashutter transistor, a CMOS photogate transistor, and an anti-bloomingtransistor.

As shown in FIGS. 3A and 3B, array 311 includes a barrier area 330.Illustratively, the barrier area 330 surrounds each one of the activearray region 12 and black region 13, black regions 313 a and 313 b (FIG.3B), as well as the array 311 as a whole. It should be understood,however, that barrier area 330 need not surround the array 311 or eachone of the active array region 12 and black region 13, black regions 313a and 313 b (FIG. 3B). Instead, the barrier area can be only adjacent toany one or more of, or any portion of, active array region 12 and blackregion 13, black regions 313 a and 313 b (FIG. 3B), or the array 311 asa whole. That is, barrier area 311 can be between any one or more of, orany portion of, active array region 12 and black region 13, blackregions 313 a and 313 b (FIG. 3B), or the array 311 and adjacentperipheral circuits, e.g., peripheral circuitry 15. Further, the barrierarea 330 can be located beneath a light blocking layer 470 (FIG. 4A),e.g., a metal a metal layer, a black color filter array, or an opaqueorganic or inorganic material, in a manner like the black region 13, andblack regions 313 a and 313 b (FIG. 3B).

The barrier area 330 includes one or more barrier regions 420 a (FIG.4A), 420 b (FIG. 4B), 420 c (FIG. 4C), and/or 420 d (FIG. 4D). Eachbarrier region 420 a, 420 b, 420 c, 420 d comprises a chargeaccumulation region 422 electrically connected to a voltage sourceterminal. In the exemplary embodiments depicted in FIGS. 3-6, the pixelcells 20 of array 311 are formed using NMOS transistors, e.g., rowselect transistor 26. Accordingly, charge accumulation region 422 is ann-type region at a surface of a substrate 501 (FIG. 5A) and is connectedto a positive voltage source terminal, e.g., Vaa-pix. In anotherembodiment, the pixel cells 20 of array 311 could include PMOStransistors, and charge accumulation region 422 would be a p-type regionconnected to a ground voltage source terminal.

Each barrier region 420 a, 420 b, 420 c, 420 d collects charge in thecharge accumulation region 422 and the charge is drained from thebarrier region to the voltage source terminal. In this manner, thebarrier regions 420 a, 420 b, 420 c, 420 d serve to isolate adjacentstructures and prevent interference from excess charge. For example, inthe embodiment of FIGS. 3A and 3B, the barrier regions 420 a (FIG. 4A),420 b (FIG. 4B), 420 c (FIG. 4B), 420 d (FIG. 4D) of barrier area 330isolate the active array region 12 from the peripheral circuitry 15.Thereby, barrier regions 420 a, 420 b, 420 c, 420 d prevent chargegenerated by the peripheral circuitry 15 from interfering with pixelcells 20 of the active array region 12. Also, the barrier regions 420 a,420 b, 420 c, 420 d isolate the black region 13, and black regions 313b, 313 c (FIG. 3B) from the active array region 12. Thereby, barrierregions 420 a, 420 b, 420 c, 420 d prevent excess charge from pixelcells 20 of the active array region 12 from interfering with the pixelcells 20 of the black region 13, and black regions 313 b, 313 c (FIG.3B).

FIGS. 4A-4D illustrate barrier regions 420 a, 420 b, and 420 c, 420 drespectively, according to exemplary embodiments of the invention, whichcan be formed by modifying pixel cells 20 to be operative for isolationrather than for the production of a pixel signal.

FIG. 4A depicts a barrier region 420 according to an exemplaryembodiment of the invention. As shown in FIG. 4A, barrier region 420 ais configured similarly to pixel cell 20. Accordingly, like pixel cell20, barrier region 420 a includes a transfer transistor 27, a resettransistor 28, a source follower transistor 29, a row select transistor26, and a floating diffusion region 25. The barrier region 420 a alsoincludes a charge accumulation region 422, which is analogous to thephotodiode charge accumulation region 22 of pixel cell 20. The barrierregion 420 a can also include an optional p-type surface layer 24 overthe charge accumulation region 422. In addition to the structure ofpixel cell 20, the barrier region 420 a includes a contact 23 and aconnection 424 through which the charge accumulation region 422 isconnected to Vaa-pix, or any positive voltage source terminal.

As shown in FIG. 4A, barrier region 420 a can be located below a lightblocking layer 470. Light blocking layer 470 can be, for example, ametal layer, a black color filter array, or an opaque organic orinorganic material. Light blocking layer 470 can be common to thebarrier layer 420 a, the black region 13 (FIG. 3A), and black regions313 a and 313 b (FIG. 3B). For simplicity, only barrier region 420A isshown below a light blocking layer 470. It should be understood,however, that any barrier region 420 a, 420 b, 420 c, 420 d can belocated below light blocking layer 470.

Except for the charge accumulation region 422, the contact 23, andconnection 424 to Vaa-pix, the other illustrated structures of barrierregion 420 a are not necessary to achieve improved isolation accordingto the invention. However, forming barrier region 420 a with a similarconfiguration to pixel cell 20 may provide advantages in the fabricationof array 311, and particularly during photolithographic, patterning, andetching processes. Printing, patterning, and etching similar featuresadjacent to the active array region 12 and black regions 13, 313 b, 313c better ensures accurate formation of the pixel cells 20 at the edgesof the active array region 12 and black regions 13, 313 b, 313 c.

For purposes of this description, when a barrier region, e.g., barrierregions 420 a (FIG. 4A), 420 c (FIG. 4B), 420 c (FIG. 4C), has astructure similar to pixel cells 20, the barrier region has aconfiguration based on the pixel cells 20. That is, the barrier regionis configured as at least a portion of a pixel cell 20 in the array 311.

As shown in FIG. 4B, barrier region 420 b has a similar configuration tothat of the pixel cell 20. Like pixel cell 20, the barrier region 420 bincludes a row select transistor 26 and a source follower transistor 29.Also, the charge accumulation region 422 is similar to the photodiodecharge accumulation region 22, except that charge accumulation region422 extends to the contact 23 connected by connection 424 to Vaa-pix.Because the charge accumulation region 422 is thus extended, thetransfer transistor 27 and the reset transistor 28 are omitted frombarrier region 420 a. As noted above, the charge accumulation region 422could instead be connected to any positive voltage source terminal.

The barrier region 420 b can also serve to ensure accurate formation ofthe pixel cells 20 at the edges of the active array region 12 and blackregions 13, 313 b, 313 c, since the features of barrier region 420 b canbe made similar to those adjacent to the active array region 12 andblack regions 13, 313 b, 313 c. Additionally, by providing an extendedcharge accumulation region 422, charge accumulation, and, therefore,isolation, can be further improved.

FIG. 4C illustrates a barrier region 420 c according to anotherexemplary embodiment of the invention. The barrier region 420 c issimilar to barrier region 420 a, except that the illustrated barrierregion 420 c includes a plurality of the FIG. 4B barrier region 420 bstructures, or, for clarity, sub-regions 444, joined with a single,continuous charge accumulation region 422. Accordingly, like barrierregion 420 b, the configuration of barrier region 420 c is based on theconfiguration of the array pixel cells 20. Therefore, the barrier region420 c includes a plurality of row select transistors 26 and a sourcefollower transistors 29; contacts 23; connections 424; and connectionsto Vaa-pix, or any positive voltage source terminal.

Illustratively, the barrier region 420 c includes three sub-regions 444joined with a single, continuous charge accumulation region 422, butbarrier region 420 c can include two or more of the sub-regions 444joined with a single, continuous charge accumulation region 422.Specifically, an entire row or column of array 311 can be formed as abarrier region 420 b. As shown in FIG. 4C, the sub-regions 444 can bestacked horizontally, in the column direction. In an alternativeembodiment, barrier region 420 c can include sub-regions 444 adjoined inother orientations, for example, in the row direction or back-to-back,such that the barrier region 420 c includes a first sub-region 444 and asecond sub-region 444 in a mirror-image orientation to the firstsub-region 444.

The barrier region 420 c can also serve to ensure accurate formation ofthe pixel cells 20 at the edges of the active array region 12 and blackregions 13, 313 b, 313 c, since the features of barrier region 420 c canbe made similar to those adjacent to the active array region 12 andblack regions 13, 313 b, 313 c. Additionally, by providing a continuouscharge accumulation region 422, charge accumulation, and, therefore,isolation, can be further improved.

FIG. 4D illustrates a barrier region 420 d according to anotherexemplary embodiment of the invention. The barrier region 422 d includesan n-type charge accumulation region 422 electrically connected toVaa-pix through a contact 23 and connection 424. Although barrier region420 d is shown having a rectangular shape with a single connection tothe array voltage source terminal Vaa-pix, the barrier region 420 d canhave any shape and any number of contacts 23 and connections 424 toVaa-pix, or any positive voltage source terminal. For example, barrierregion 420 d can have a shape similar to a pixel cell 20, or to a groupof pixel cells 20, e.g. one or more rows and/or columns of pixel cells20, having a continuous charge accumulation region 422. Also, thebarrier area 330 can include a single barrier region 420 d throughoutapproximately the entire barrier area 330 (FIGS. 3A and 3B). Byproviding a continuous charge accumulation region 422, chargeaccumulation, and, therefore, isolation, can be further improved.

As noted above, embodiments of the invention can include pixel cellshaving alternative configurations, e.g., 3T, 5T, 6T, and 7Tconfigurations. Accordingly, when the array 311 includes pixel cellshaving an alternative configuration, the image sensor 300 a, 300 b caninclude barrier regions based on such an alternative configuration. Insuch a case, the alternatively configured pixel cell could be modifiedby eliminating structures and/or connecting a charge accumulation regionto a voltage source terminal.

FIGS. 5A-5F depict a portion of the barrier region 420 a along line4A-4A′, and FIGS. 5G and 5H depict a portion of the barrier region 420 aalong line 4AA-4AA′. FIGS. 5A-5H illustrate the formation of barrierregion 420 a (FIG. 4A) according to one exemplary embodiment of theinvention. No particular order is required for any of the actionsdescribed herein, except for those logically requiring the results ofprior actions. Accordingly, while the actions below are described asbeing performed in a general order, the order is exemplary only and canbe altered.

Referring to FIGS. 4A and 5A-5H, the barrier region 420 a can be formedsimultaneously with the pixel cells of array 311 (FIGS. 3 a and 3B).Also, the formation of a plurality of barrier regions 420 a can proceedsimultaneously and in a similar manner as described below in connectionwith FIGS. 5A-5H.

As shown in FIG. 5A, barrier region 420 a is formed at a surface of asubstrate 501, which is illustratively a p-type substrate. As notedabove, previous process steps may have been utilized to form regions(not shown) or junctions (not shown) in the substrate 501. For example,isolation regions, e.g., shallow trench isolation regions can be formedby known techniques at a surface of the substrate 501 prior to theformation of barrier region 420 a.

FIG. 5B depicts the formation of gate stacks 550 for the transfer 29 andreset transistor 28 gates. A first insulating layer 550 a of, forexample, silicon oxide is grown or deposited on the substrate 501. Thefirst insulating layer 550 a serves as the gate oxide layer for thesubsequently formed transfer and reset transistors 27, 28. Next, a layerof conductive material 550 b is deposited over the oxide layer 550 a.The conductive layer 550 b serves as the gate electrode for thesubsequently formed transfer and reset transistors 27, 28. Theconductive layer 550 b can be a layer of polysilicon, which can ben-type. A second insulating layer 550 c is deposited over thepolysilicon layer 550 b. The second insulating layer 550 c can be formedof, for example, an oxide (SiO₂), a nitride (silicon nitride), anoxynitride (silicon oxynitride), ON (oxide-nitride), NO (nitride-oxide),or ONO (oxide-nitride-oxide).

The layers 550 a, 550 b, 550 c, can be blanket formed by conventionaldeposition methods, such as chemical vapor deposition (CVD) or plasmaenhanced chemical vapor deposition (PECVD), among others. The layers 550a, 550 b, 550 c are then patterned and etched to form the transfer andreset transistor 27, 28 multilayer gate stacks 550 shown in FIG. 5B. Theformation of gate stacks 550 for barrier region 420 a can be formedsimultaneously with the analogous structures of pixel cells 20.

The invention is not limited to the structure of the gate stacks 550described above. Additional layers may be added or the gate stacks 550may be altered as is desired and known in the art. For example, asilicide layer (not shown) may be formed between the gate electrodes 550b and the second insulating layers 550 c. The silicide layer may beincluded in the transfer and reset transistor gate stacks 550, or in allof the transistor gate structures in an image sensor circuit, and may betitanium silicide, tungsten silicide, cobalt silicide, molybdenumsilicide, or tantalum silicide. This additional conductive layer mayalso be a barrier layer/refractor metal, such as TiN/W or W/N_(x)/W, orit could be formed entirely of WN_(x).

Optionally, as illustrated in FIG. 5C, a p-type well 505 is implantedinto the substrate 501. The p-well 505 is formed in the substrate 501from a point below the transfer transistor 27 gate stack 550 andextending below the reset transistor 27 gate stack 550. The p-well 505may be formed by known methods. For example, a layer of photoresist (notshown) can be patterned over the substrate 501 having an opening overthe area where a p-well 505 is to be formed. A p-type dopant, such asboron, can be implanted into the substrate 501 through the opening inthe photoresist. Illustratively, the p-well 505 is formed having ap-type dopant concentration that is higher than adjacent portions of thesubstrate 501.

As depicted in FIG. 5D, the charge accumulation region 422 is implantedin the substrate 501. The charge accumulation region 422 is,illustratively, a lightly doped n-type region. The implant dose in thecharge accumulation region is within the range of approximately 5×10¹¹to approximately 1×10¹⁴ atoms/cm², and preferably is within the range ofapproximately 2×10¹² atoms/cm² to approximately 1×10¹⁴ atoms/cm². Inanother embodiment, the charge accumulation region 422 can be a heavilydoped n+ region, and the implant dose can be greater than approximately1×10¹⁴ atoms/cm². For example, a layer of photoresist (not shown) may bepatterned over the substrate 501 having an opening over the surface ofthe substrate 501 where charge accumulation region 422 is to be formed.An n-type dopant, such as phosphorus, arsenic, or antimony, may beimplanted through the opening and into the substrate 501. Multipleimplants may be used to tailor the profile of region 422. If desired, anangled implantation may be conducted to form the region 422, such thatimplantation is carried out at angles other than 90 degrees relative tothe surface of the substrate 501. The charge accumulation region 422 canbe formed simultaneously with the photodiode charge accumulation regions22 of pixel cells 20.

The floating diffusion region 25 and the reset transistor 28source/drain region 60 are implanted by known methods to achieve thestructure shown in FIG. 5D. The floating diffusion region 25 andsource/drain region 60 are formed as n-type regions adjacent the gatestacks 550. The floating diffusion region 25 is formed between thetransfer transistor 27 gate stack 550 and the reset transistor 28 gatestack 550, while the source/drain region 60 is formed adjacent to thereset transistor 28 gate stack 550 and opposite to the floatingdiffusion region 25. Any suitable n-type dopant, such as phosphorus,arsenic, or antimony, may be used. The source/drain region 60 can beformed simultaneously with source/drain regions 60 of pixel cells 20.

Referring to FIG. 5E, a dielectric layer 506 is formed over thesubstrate 501 and the gate stacks 550. Illustratively, layer 506 is anoxide layer, but layer 506 may be any appropriate dielectric material,such as silicon dioxide, silicon nitride, an oxynitride, ON, NO, ONO, orTEOS, among others, formed by methods known in the art.

As depicted in FIG. 5F, a dry etch step is conducted to etch portions ofthe oxide layer 506 such that only sidewall spacers 506 on gate stacks550 remain. Alternatively, oxide layer 506 may be etched such thatremaining portions form a sidewall spacer 506 on a sidewall of resettransistor 28 gate stack 550 and a protective layer (not shown)extending from the top of reset transistor 28 gate stack 550 over thetransfer transistor 27 gate stack 500 and the charge accumulation region422.

Optionally, a p-type surface layer 24, analogous to the p-type surfacelayer 24 of the photodiode 21 of pixel cell 20 (FIG. 2A), can beimplanted, as shown in FIG. 5F. The doped surface layer 24 is doped tothe first conductivity type. Illustratively, doped surface layer 24 is ahighly doped p+ surface layer. A p-type dopant, such as boron, indium,or any other suitable p-type dopant, may be used to form the p+ surfacelayer 24.

The p+ surface layer 24 may be formed by known techniques. For example,layer 24 may be formed by implanting p-type ions through openings in alayer of photoresist. Alternatively, layer 24 may be formed by a gassource plasma doping process, or by diffusing a p-type dopant into thesubstrate 501 from an in-situ doped layer or a doped oxide layerdeposited over the area where layer 24 is to be formed.

Conventional processing methods may be used to complete the barrierregion 420 a. For example, a light blocking layer 470 of a metal layer,a black color filter array, or an opaque organic or inorganic materialcan be formed by known techniques over the barrier region 420 a toprevent light from reaching the barrier layer 420 a. Additionally,insulating, shielding, and metallization layers can be formed to connectgate lines and provide connection 424 to Vaa-pix and other connectionsto the barrier region 420 a. Further, the entire surface may be coveredwith a passivation layer (not shown) of, for example, silicon dioxide,BSG, PSG, or BPSG, which is CMP planarized and etched to provide contactholes, which are then metallized to provide contacts 23. Conventionallayers of conductors and insulators may also be used to interconnect thestructures and to connect the charge accumulation region 422 to Vaa-pix.Specifically, connection 424 can be formed using any suitable conductivematerial, e.g. metal; and contact 23 can be formed using any suitableconductive material.

When barrier region 420 a includes an optional p-type surface layer 24,an n-type contact region 423 can be formed to provide the contact 23 aconnection to the charge accumulation region 422. FIGS. 5G and 5H arecross sectional views of a portion of the barrier region 420 a alongline 4AA-4AA′ showing the formation of the contact region 423.

FIG. 5G shows a portion of the barrier region after the processing stepsdescribed above in connection with FIG. 5F. Accordingly, at a surface ofthe substrate 501, there is an n-type charge accumulation region 422below a p-type surface layer 24.

As depicted in FIG. 5H, an n-type contact region 423 is formed at asurface of the substrate 501 at a location where contact 23 to Vaa-pixis to be formed and extending from the charge accumulation region 422 tothe surface of the substrate 501. The contact region 423 can be formedby known techniques. For example, a layer of photoresist (not shown) maybe patterned over the substrate 501 having an opening over the surfaceof the substrate 501 where contact region 423 is to be formed. An n-typedopant, such as phosphorus, arsenic, or antimony, may be implantedthrough the opening and into the substrate 501. The implant dose in thecontact region 423 can be approximately the same implant dose as usedfor the charge accumulation region 422 or a different implant dose.Illustratively, the contact region is an n+ region and the implant doseis greater than approximately 1×10¹⁴ atoms/cm².

When barrier region 420 a is formed simultaneously with pixel cells 20,barrier region 420 a can be shielded from certain processing stepsperformed on pixel cells 20 by, for example a mask, such as photoresist.In this manner, barrier region 420 a can be formed with a configurationas desired. For example, if pixel cells 20 are subjected to a p-typedopant implantation step, to form a p-type region, e.g., a p-well 505,the barrier region 420 a can be shielded from this step.

The barrier regions 420 b (FIG. 4B), 420 c (FIG. 4C) and 420 d (FIG. 4D)can be formed in a similar manner as described above in connection withFIGS. 5A-5D. To form barrier regions 420 b, 420 c, 420 d particularsteps described above in connection with FIGS. 5A-5D can be omitted orbarrier regions 420 b, 420 c, 420 d can be shielded from the particularsteps.

FIG. 6 illustrates a processor-based system 600 including an imagesensor 300 a of FIG. 3A. In an alternative embodiment, the processorbased system 600 can include the image sensor 300 b of FIG. 3B. Theprocessor-based system 600 is exemplary of a system having digitalcircuits that could include image sensor devices. Without beinglimiting, such a system could include a computer system, camera system,scanner, machine vision, vehicle navigation, video phone, surveillancesystem, auto focus system, star tracker system, motion detection system,image stabilization system, and data compression system.

The processor-based system 600, for example a camera system, generallycomprises a central processing unit (CPU) 660, such as a microprocessor,that communicates with an input/output (I/O) device 661 over a bus 663.Image sensor 300 a also communicates with the CPU 660 over bus 663. Theprocessor-based system 600 also includes random access memory (RAM) 662,and can include removable memory 664, such as flash memory, which alsocommunicate with CPU 660 over the bus 663. Image sensor 300 a may becombined with a processor, such as a CPU, digital signal processor, ormicroprocessor, with or without memory storage on a single integratedcircuit or on a different chip than the processor.

It is again noted that the above description and drawings are exemplaryand illustrate preferred embodiments that achieve the objects, featuresand advantages of the present invention. It is not intended that thepresent invention be limited to the illustrated embodiments. Anymodification of the present invention which comes within the spirit andscope of the following claims should be considered part of the presentinvention.

1. A method of forming a barrier region for isolating devices of animage sensor comprising a pixel array having an active array regioncomprising at least one active pixel cell, and a black region comprisingat least one pixel cell not in the active array region, the methodcomprising: forming a group of barrier pixel cells in the array having acommon charge accumulation region of a particular conductivity type in asubstrate adjacent to the at least one active pixel cell, wherein thegroup of barrier pixel cells is formed surrounding the at least oneblack region and between the black region and the active array region;and electrically connecting the charge accumulation region to a voltagesource terminal.
 2. The method of claim 1, wherein the act of formingthe charge accumulation region comprises forming either an np or a pnpdiode.
 3. The method of claim 1, wherein the act of forming the chargeaccumulation region comprises forming the charge accumulation regionwith n-type conductivity, and wherein the act of electrically connectingthe charge accumulation region comprises electrically connecting thecharge accumulation region to a positive voltage source terminal.
 4. Themethod of claim 3, wherein the act of forming the charge accumulationregion comprises forming the charge accumulation region using an n-typeimplant dose within the range of approximately 5×10¹¹ to approximately1×10¹⁴ atoms/cm².
 5. The method of claim 1, wherein the act of formingthe charge accumulation region comprises forming the charge accumulationregion with p-type conductivity, and wherein the act of electricallyconnecting the charge accumulation region comprises electricallyconnecting the charge accumulation region to a ground voltage sourceterminal.
 6. The method of claim 1, wherein the act of connecting thecharge accumulation region to a voltage source terminal comprisesforming a contact and a metal connection, and the act of forming thebarrier region comprises forming a contact region of the particularconductivity type below the contact.
 7. The method of claim 6, whereinthe act of forming the contact region comprises forming the contactregion using an n-type implant dose greater than approximately 1×10¹⁴atoms/cm2.
 8. The method of claim 1, wherein the act of forming thebarrier region comprises forming the barrier region as either a row orcolumn of pixel cells.
 9. The method of claim 1, wherein the group ofbarrier pixel cells is formed between the at least one active pixel celland peripheral circuitry of the image sensor.
 10. The method of claim 1,wherein the group of barrier pixel cells is formed between the blackregion and peripheral circuitry of the image sensor.
 11. A method offorming an image sensor, the method comprising: providing a substrate;providing an array of pixel cells on the substrate comprising an activearray region comprising a first portion of pixel cells, and at least oneblack region comprising a second portion of pixel cells not in theactive array region; and forming at least one barrier region adjacent toat least one pixel cell by forming a charge accumulation region of aparticular conductivity type on the substrate and electricallyconnecting the charge accumulation region to a voltage source terminal,the act of forming the at least one barrier region further comprisingforming the at least one barrier region surrounding the at least oneblack region and between the black region and the active array region.12. The method of claim 11, wherein the act of forming the chargeaccumulation region comprises forming the charge accumulation regionwith n-type conductivity, and wherein the act of electrically connectingthe charge accumulation region comprises electrically connecting thecharge accumulation region to a positive voltage source terminal. 13.The method of claim 11, wherein the act of forming the chargeaccumulation region comprises forming the charge accumulation regionwith p-type conductivity, and wherein the act of electrically connectingthe charge accumulation region comprises electrically connecting thecharge accumulation region to a ground voltage source terminal.
 14. Themethod of claim 11, further comprising peripheral circuitry adjacent tothe array, wherein the act of forming the at least one barrier regioncomprises forming the at least one barrier region between the array andthe peripheral circuitry.
 15. The method of claim 11, wherein the act ofproviding the array comprises providing the array comprising a firstblack region adjacent to a first side of the active array region and atleast a second black region adjacent to a second side of the activearray region, the first and at least second black regions fordetermining the black level of the array, and wherein the act of formingthe at least one barrier region comprises forming the at least onebarrier region between the first black region and the active arrayregion and between the at least second black region and the active arrayregion.
 16. The method of claim 11, wherein the act of forming the atleast one barrier region comprises forming the at least one barrierregion surrounding the active array region.
 17. A method of forming animage sensor, the method comprising: providing an array of pixel cellson a substrate comprising an active array region comprising a firstportion of pixel cells, and at least one black region comprising asecond portion of pixel cells not in the active array region; providingperipheral circuitry adjacent to the array; and forming a plurality ofbarrier regions by forming a plurality of n-type charge accumulationregions on the substrate and electrically connecting each of the chargeaccumulation regions to a positive voltage source terminal, each barrierregion being formed adjacent to at least one pixel cell, the act offorming the plurality of barrier regions further comprising forming atleast one barrier region surrounding the black region and between theblack region and the peripheral circuitry.
 18. The method of claim 17,wherein the act of forming the plurality of barrier regions comprisesforming a first portion of the barrier regions between the array and theperipheral circuitry and forming a second portion of the barrier regionsbetween two or more pixel cells.
 19. The method of claim 17, wherein theact of forming the plurality of barrier regions comprises forming one ormore of the barrier regions as at least a portion of a pixel cell in thearray.
 20. The method of claim 17, wherein the act of forming theplurality of barrier regions further comprises forming at least onebarrier region between the black region and the active array region.